With the rapid development of digitalization of information, digitalization in image processing is increasingly required. In digital cameras in particular, solid-state image pickup devices, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been used instead of film.
In image pickup apparatuses including CCDs or CMOS sensors, an image of an object is optically taken by an optical system and is extracted by an image pickup device in a form of an electric signal. In an image pickup apparatus, light is regularly dispersed by a phase plate and is reconstructed by digital processing techniques to achieve a large depth of field. In addition, a filtering process using a transfer function is performed in an automatic exposure control system for a digital camera.
Devices such as CCD and CMOS sensors that have image input functions sometimes read close-up still images, such as bar codes, together with desired images, such as landscape images. Techniques used for reading bar codes comprise an auto-focus technique in which focusing is performed by moving a lens towards and away from the bar code. In this case, a depth expansion technique is used in which the depth of field is increased by reducing the f-number in a camera so as to achieve a corrected focus.
In some image pickup apparatuses, a Point Spread Function (PSF) obtained is constant when the phase plate mentioned above is placed in the optical system. The PSF describes the response of an imaging system to a point source or point object. The degree of spreading (blurring) of the point object is a measure for the quality of an imaging system. If the PSF varies, it can be difficult to obtain an image with a large depth of field by convolution using a kernel.
Therefore, in lens systems (excluding single focus lens systems) such as zoom systems and autofocus (AF) systems, high precision is required in the optical design, thereby increasing costs accordingly. In an automatic exposure control system for a digital camera, filtering is performed using a transfer function. More specifically, in known image pickup apparatuses, a suitable convolution operation cannot be performed and the optical system should be designed to eliminate aberrations, such as astigmatism, coma aberration, and zoom chromatic aberration that cause a displacement of a spot image at wide angle and telephoto positions. However, eliminating the aberrations can increase the complexity of an optical design, the number of design steps, the costs, and the lens size.
The above-described techniques are premised on image restoration, and therefore increase noise and cost resulting from image processing. Further, since the optical transfer function (OTF) should not depend on an object distance in image restoration, it is necessary to design an optical system whose OTF remains constant even in an out-of-focus state. Even if depth extension is realized for an f-number having a certain characteristic, in general a depth extending function is lost or its efficiency significantly decreases when the aperture diameter changes.
Accordingly, there is a need for an image pickup apparatus that achieves depth extension without performing image restoration and that prevents the depth extending function from being impaired when the aperture diameter changes.